This invention relates to downhole well tools and more particularly to downhole chemical cutting tools.
There are many circumstances in the oil industry where it is desirable to cut or completely sever downhole tubular goods within a well. For example, in the course of drilling a well, drill pipe may become stuck or "frozen" at a location well below the surface of the earth. This may result, for example, from "key seating" in which a drill collar or other section of the drill string becomes lodged against the side of the well, or the drill string may become stuck as a result of cuttings which settle within the well around the lower portion of the drill string. Similarly, in the completion or operation of oil or gas wells, it is often necessary to carry out downhole cutting operations. For example, it may be desirable to sever casing or tubing at a downhole location in order to make repairs or to withdraw the tubing or casing from a well which is being abandoned.
In other circumstances, it is desirable to cut slots, grooves or perforations in downhole tubular goods. For example the perforating of the casing and the surrounding cement sheath to provide fluid access to a hydrocarbon bearing formation is a conventional expedient. Similarly it is often desirable to perforate tubing in the completion or recompletion of a well.
While mechanical means may be employed to cut openings or to completely sever downhole tubular goods, this is often accomplished through chemical cutting techniques. Many times shaped charges are employed to perforate or sever tubular goods within the well. However, another technique which can often be used to great advantage is the application of a chemical which cuts through metal tubular goods in the well by direct chemical reaction. For example U.S. Pat. No. 2,918,125 to Sweetman discloses a downhole chemical cutter in which halogen fluorides are employed in jet cutting streams. The attendant reaction is highly exothermic and the tubing, drill pipe, etc. is rapidly penetrated.
During the course of the cutting operation, it is desirable to anchor the cutting tool at the desired location within the well. This is particularly the case where the cutting tool is run into the well on a wire line. One technique for anchoring the tool employs use of fluid pressure from a suitable source to both activate the anchoring means and to dispell cutting fluid from the tool against the surface to be severed or otherwise cut. For example, U.S. Pat. No. 4,125,161 to Chammas discloses a cutting tool in which gas from a propellant charge displaces a piston to cam one or more wedges outwardly against the tubing string or other object to be cut. The gas from the propellant charge is also employed to force the cutting chemical into contact with a preignitor and thence downwardly through the bore of a severing head. The severing head is provided with discharge parts through which the chemical issues, and circulation ports at the lower end of the severing-head bore. A piston in the bore is disposed adjacent to the discharge ports, but does not close these ports with respect to the lower portion of the bore and the circulation ports. The chemical under pressure forces the piston downwardly until it abuts against a shoulder formed by a reduction in the severing head bore. In this portion, the piston is below the discharge ports and the chemical flows outwardly through these ports into contact with the tubular goods.
A particularly effective chemical cutting tool is disclosed in U.S. patent application Ser. No. 078,472 filed Sept. 24, 1979 by Jamie B. Terrell. In this tool a chemical module assembly is located intermediate a propellant and slip assembly and a discharge head assembly. Gas pressure generated by the ignition of a propellant charge is employed to rapidly move a slip array against a slip expander, during which time the cutting action takes place. The slip array is then rapidly retracted by means of a biasing mechanism. The gas pressure also forces chemical from the chemical module assembly into the discharge head assembly having a central bore which is equipped with discharge ports similarly as described above, but which is closed at its lower end. The chemical forces a piston, which normally closes the discharge ports, downwardly, thus opening the ports to the chemical.